1. Field
The present disclosure relates to light emitting devices, and more particularly, to light emitting devices with constant forward voltage.
2. Background
Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources, such as incandescent lamps and fluorescent light sources. LEDs have substantially higher light conversion efficiencies than incandescent lamps and longer lifetimes than both types of conventional light sources. In addition, some types of LEDs now have higher conversion efficiencies than fluorescent light sources and still higher conversion efficiencies have been demonstrated in the laboratory. Furthermore, LEDs require lower voltages than fluorescent lamps, and therefore, are better suited for applications in which the light source must be powered from a low-voltage source, such as a battery or an internal computer DC power source.
Unfortunately, LEDs produce light in a relatively narrow spectrum band. To replace conventional lighting sources, LEDs that generate light that appears to be “white” to the human observer are required. A light source that appears to be white and that has a conversion efficiency comparable to that of fluorescent light sources can be constructed from a blue LED that is covered with a layer of phosphor that converts a portion of the blue light to yellow light. These LEDs will be referred to as “white LEDs”.
White LEDs are based on wide band gap semiconductors, such as InGaN (indium gallium nitride) and the like. These semiconductors emit light when electrically biased by a voltage (Vf) in the forward direction of the p-n junction. To achieve the brightness that is required for many applications, these semiconductors are generally driven at the maximum rated current, and therefore, are designed with the lowest possible forward voltage Vf to minimize power consumption. This poses several challenges to one skilled in the art when multiple LEDs with different forward voltages Vf are required to be connected together in parallel. If one LED has a significantly lower resistance in such an arrangement, it will draw more current until it gets so hot that its internal resistance rises to match that of the other LEDs. This heating leads to many undesirable effects, such as color shift, different light output and other electrical and thermal non-uniformities. To avoid these problems, LEDs have to be paired with resistors so that each LED/resistor pair in the parallel arrangement has a constant ohmic value. Another approach is to carefully select or “bin” the LEDs for similar values of Vf.
Recent advances in LED technology have resulted in a new generation of the LEDs that provide significantly more light. As a result, skilled artisans may be ready to trade some of the light output for a constant forward voltage Vf, thus eliminating the need for a series current limiting resistor or binning.